Properly working post increment/decrement operators

Properly working post increment/decrement operators

This required some larger changes in our infrastructure.
Unfortunately the post increment/decrement operators
don't return the old value unmodified, but return
toNumber(oldValue).

At the same time they need to properly store the new value
into the referenced expression. The only way to solve this
(as we can't have two return values) is to pass a proper
reference into runtime methods.

Change-Id: I0e0c2cc011ab22d5d4b27924abc8c18372c104a5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>